1. Field of the Invention
The present invention relates to a method of assembling and adjusting a multi-beam scanning optical apparatus and a method of manufacturing a multi-beam scanning optical apparatus, and more particularly, to a method of assembling and adjusting a multi-beam scanning optical apparatus and a method of manufacturing a multi-beam scanning optical apparatus, which are suitable for a multi-beam scanning optical apparatus used for an image forming apparatus employing an electrophotographic process, such as a laser beam printer, a digital copy machine, and a multi-function printer.
2. Description of the Related Art
In a scanning optical apparatus of an image forming apparatus such as a laser beam printer (LBP), conventionally, a light beam emitted from a light source unit, which is optically modulated in accordance with an image signal, is periodically deflected by a deflecting unit including a rotating polygon mirror. The light beam that has undergone deflection scanning is then focused on a surface of a photosensitive recording medium (photosensitive drum) in a spot shape by an imaging optical system having an fθ characteristic to optically scan the surface, thus recording an image on the surface of the photosensitive recording medium.
Under increasing need for a printing machine having a high printing speed, adequate measures taken in the scanning optical apparatus include, for example, (1) increasing the rotation speed of a deflecting unit (such as the polygon mirror), (2) increasing the number of mirror planes of the polygon mirror, and (3) increasing the number of light beams (using a multi-beam laser, combining light beams by using a prism, or causing multiple light beams to be entered into a polygon mirror at different angles in a deflecting direction). However, the measures (1) and (2) raise such problems as a limit of the rotation speed and a restriction of a size of the polygon mirror due to the increase of the number of the mirror planes thereof, respectively, and hence a multi-beam scanning optical apparatus using a multi-beam light source of the measure (3) is increasingly used.
However, in the multi-beam scanning optical apparatus, a scanning interval misalignment or a scanning position misalignment (misregistration) in a sub-scanning direction of the multi-beam may occur due to a mounting error of each optical element. To cope with this problem, Japanese Patent Application Laid-Open No. H10-319336 discloses a method of adjusting a sub-scanning interval of the multi-beam on a surface of a photosensitive drum, in which a semiconductor laser is rotated about a center axis. In addition, although a multi-beam laser is not employed, Japanese Patent Application Laid-Open No. H11-326804 discloses an adjustment method of suppressing a misregistration in a sub-scanning direction between different colors by turning a diffractive optical element in a color image forming apparatus including multiple scanning optical apparatus. Further, as disclosed in Japanese Patent Application Laid-Open No. 2004-258182, a method of correcting a misregistration of a scanning line in a sub-scanning direction has been known, in which an imaging optical element is moved in the sub-scanning direction.
However, when the methods described in Japanese Patent Application Laid-Open No. H11-326804 and Japanese Patent Application Laid-Open No. 2004-258182 are applied to correct the scanning position misalignment (misregistration) in the multi-beam scanning optical apparatus, there occurs a problem that the sub-scanning interval between beams of the multi-beam on the surface of the photosensitive drum becomes non-constant relative to an image height. This phenomenon is caused by a fact that, when the imaging optical element is moved to correct the scanning position misalignment (misregistration) in the sub-scanning direction, a sub-scanning position on the imaging optical element is largely deviated from a position on which the multiple laser beams are supposed to be incident.